Gratuitous ARP, is the ARP that is used to inform the network about IP to MAC Mappings after a change. It is sent as Broadcast message to all the nodes in the network. The usage of Gratuitous ARP is a little diffeerent than ARP (Address Resolution Protocol) and Proxy ARP. Because of network configuration changes or any failure, Gratuitous ARP is used to inform the network nodes about the situation after this change. There can be various scenarious for this.
For example, After a manual MAC address change, Gratuitous ARP provide to inform other nodes about the new MAC address. And the IP to MAC Mapping. As you can see below, at the beginning, IP to MAC Mapping of the devices are know.
G ARP is also used to inform the network as an assistant to network redundancy mechanims. There can be multiple devices that uses same IP address. After one of the devices in the redundancy mechanims fails, with the help of Gratuitous ARP, network nodes are informed about the new device’s MAC address. IP to MAC mapping is updated.
We can give an example to this situation. As you can see below, the IP to MAC mapping is like below at the beginning of this network that uses a redundancy mechanism.